Laser light sources are commonly used for medical applications, and are particularly effective for rapid, non-invasive surgical procedures. In one application, optical radiation from the light source is used to irradiate the tissue, where it is rapidly absorbed and converted into heat. The heated regions of tissue can be ablated in this manner, resulting in resection or incision of the tissue.
In a particular example, infrared lasers have recently been used during the transurethral resection of the prostate (the TURP procedure) as a way of treating benign prostatic hypertrophy. In this procedure, the infrared laser light is first coupled into a fiber optic waveguide, which is then fed through the urinary tract using a catheter. The fiber is positioned within the prostrate urethra and then used to irradiate and resect the hypertrophic region. The most common laser which has been used for this procedure is the Nd:YAG laser, which emits light at .lambda.=1.06 microns. Light at this wavelength is not strongly absorbed, and thus deeply penetrates the tissue to produce only small amounts of ablation, while creating a wide region of coagulation necrosis. Alternatively, radiation from a holmium:YAG laser (.lambda.=2.1 microns) is typically strongly absorbed by the tissue, and can be used to effectively ablate the irradiated region while leaving a relatively small region of coagulation necrosis.
Unfortunately, while infrared radiation is rapidly absorbed by water contained within the tissue, it is also absorbed by water surrounding the tissue, leading to an attenuation of the incising optical field. Absorption (and heating) of the water thus decreases the amount of radiation available for the surgical procedure, and may also lead to thermal lensing effects in the surrounding fluid which can reduce the accuracy of the procedure.
One technique used to reduce the absorption of infrared radiation in fluids, such as water, is the "Moses effect". Here, a first infrared optical pulse is used to irradiate a liquid region separating the tissue from, for example, the delivery end of an optical fiber. Absorption of the pulse leads to rapid heating of the fluid, creating a vapor bubble (i.e., an air or steam pocket surrounded by liquid) which expands outwardly in all directions. Once the bubble reaches a desirable size during a predetermined time period, a second laser pulse is delivered from the fiber and propagates through the bubble before reaching the tissue, thereby minimizing the absorption and attenuation by the surrounding fluid.
The amount of energy necessary to vaporize the liquid in order to create the vapor bubble is dependent on the absorbing properties of the liquid, and the carrier frequency, pulse duration, spot size, and pulse energy of the optical field. Other properties of the bubble, such as the volume and inflation lifetime, are additionally dependent on the optical properties of the fluid and the various parameters of the irradiating field. Typically, using water as the liquid and a Nd:YAG laser as the light source, bubbles having a volume of up to a few cubic centimeters can be formed during a time period of between 100 and 200 microseconds.
U.S. Pat. No. 5,321,715 (issued Jun. 14, 1994; assigned to Coherent, Inc.) describes the use of a pair of optical pulses to reduce the absorption effects of a liquid medium surrounding a tissue sample. In this device, a first pulse is used to generate the vapor bubble; a second pulse, emitted from the same fiber, propagates through the bubble to irradiate the tissue.